1. Field of the Invention
This invention relates to a communication system of the multiple access type which is constituted from a master station and a plurality of slave stations, a broadcasting channel which allows communication from the master station to all of the slave stations, and a multiple access channel which allows communication from all of the slave stations to the master station, and more particularly to a communication system which can accommodate constant bit rate data and variable bit rate data.
2. Description of the Related Art
A polling system wherein a master station successively sends transmission instructions to a plurality of slave stations and the slave stations successively send out information in response to the instructions is well known as one of communication systems of the multi-point type. Communication systems of the multiple access type are disclosed, for example, in Japanese Patent Laid-Open Application No. Showa 62-21331 and Japanese Patent Laid-Open Applications Nos. Showa 62-23236 to 62-23239.
In addition, as a system which accommodates constant bit rate data and variable bit rate data in communication of the multiple access type, a system is disclosed, for example, in IEEE Transactions on Communications, Vol. 37, No. 8, pp. 885-889, Aug. 1989, wherein a slave station acquires the transmission right for variable bit rate data and constant bit rate data in accordance with a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) system and starts transmission of constant bit rate data. Once the slave station acquires a slot, the master station allows the slave station to thereafter access the slot, which is at the position of each transmission period of the constant bit rate data as counted from the acquired slot, until the transmission of the constant bit rate data is completed. At the same time, the master station informs all of the other slave stations of the restriction of access to the slot.
In the conventional access system, transmission of constant bit rate data from a slave station sometimes collides upon starting with constant bit rate data or variable bit rate data from another slave station, and the slave station acquires the transmission right only after some delay of time. Further, since the rate of occurrence of collisions increases as the traffic increases, there is another problem in that the utilization efficiency of the circuit is low.